VPNs in their basic sense are legal in China, many large companies provide them/use them and so on. VPNs designed to bypass the Firewall without government approval are a subset of VPNs which the police do not like.
I would assume EasyTier devs use it to connect their devices within China so the great firewall isn't involved. Attempts to cross the firewall with EasyTier are detectable without things like Tor's pluggable censorship evasion transports.